Following these observations, several studies have explored the effects of surfactants on SHS drag reduction.
[1]} combined fluorescence correlation spectroscopy with numerical simulations to examine the effective slip length in a channel flow with cylindrical pillars.
They found that the effective slip length reduced when compared to a surfactant-free channel; the local slip length exhibited strong variations across the cavity, which was consistent with the Marangoni stresses which could arise in their geometry.
[2]} performed experiments and numerical simulations in a channel with longitudinal ridges, demonstrating that low levels of surfactant could yield large changes in interfacial conditions.
In particular, they showed that liquid–gas interfaces could be rendered completely no-slip at surfactant concentrations well below typical environmental values.
They used channel-flow experiments where the imposed pressure gradient was removed abruptly to show that the observed reverse flow was only possible due to adverse surfactant gradients out of equilibrium.
[3]} performed experiments for finite longitudinal ridges and infinite concentric ridges.
For finite longitudinal ridges, surface tension gradients arose due to the presence of downstream transverse contact lines, increasing the drag when compared to infinite concentric ridges, where there were no stagnation points to generate surface tension gradients.
